- DIGITAL CAMERA SENSORS -

A digital camera uses a sensor array of millions of tiny pixels in order
to produce the final image. When you press your camera's shutter button
and the exposure begins, each of these pixels has a "photosite" which is uncovered
to collect and store photons in a cavity. Once the exposure finishes,
the camera closes each of these photosites, and then tries to assess how many
photons fell into each. The relative quantity of photons in each cavity
are then sorted into various intensity levels, whose precision is determined
by bit depth (0 - 255 for an 8-bit
image).

Each cavity is unable to distinguish how much of each color has fallen in,
so the above illustration would only be able to create grayscale images.
To capture color images, each cavity has to have a filter placed over it which
only allows penetration of a particular color of light. Virtually all
current digital cameras can only capture one of the three primary colors in
each cavity, and so they discard roughly 2/3 of the incoming light. As
a result, the camera has to approximate the other two primary colors in order
to have information about all three colors at every pixel. The most common
type of color filter array is called a "Bayer array," shown below.

Color Filter Array

A Bayer array consists of alternating rows of red-green and green-blue filters.
Notice how the Bayer array contains twice as many green as red or blue sensors.
Each primary color does not receive an equal fraction of the total area because
the human eye is more sensitive to green light than both red and blue light.
Redundancy with green pixels produces an image which appears less noisy and
has finer detail than could be accomplished if each color were treated equally.
This also explains why noise in the green channel is much less than for the
other two primary colors (see "Understanding
Image Noise" for an example).

Original Scene
(shown at 200%)

What Your Camera Sees
(through a Bayer array)

Note: Not all digital cameras use a Bayer array, however this is by far the
most common setup. The Foveon sensor used in Sigma's SD9 and SD10 captures
all three colors at each pixel location. Sony cameras capture four colors
in a similar array: red, green, blue and emerald green.

BAYER DEMOSAICING

Bayer "demosaicing" is the process of translating this Bayer array of primary
colors into a final image which contains full color information at each pixel.
How is this possible if the camera is unable to directly measure full color?
One way of understanding this is to instead think of each 2x2 array of red,
green and blue as a single full color cavity.

—>

This would work fine, however most cameras take additional steps to extract
even more image information from this color array. If the camera treated
all of the colors in each 2x2 array as having landed in the same place, then
it would only be able achieve half the resolution in both the horizontal and
vertical directions. On the other hand, if a camera computed the color
using several overlapping 2x2 arrays, then it could achieve a higher resolution
than would be possible with a single set of 2x2 arrays. The following
combination of overlapping 2x2 arrays could be used to extract more image information.

—>

Note how we did not calculate image information at the very edges of the
array, since we assumed the image continued on in each direction. If these
were actually the edges of the cavity array, then calculations here would be
less accurate, since there are no longer pixels on all sides. This is
no problem, since information at the very edges of an image can easily be cropped
out for cameras with millions of pixels.

Other demosaicing algorithms exist which can extract slightly more resolution,
produce images which are less noisy, or adapt to best approximate the image
at each location.

DEMOSAICING ARTIFACTS

Images with small-scale detail near the resolution limit of the digital sensor
can sometimes trick the demosaicing algorithm—producing an unrealistic looking
result. The most common artifact is moiré (pronounced "more-ay"), which
may appear as repeating patterns, color artifacts or pixels arranges in an unrealistic
maze-like pattern:

Two separate photos are shown above—each at a different magnification.
Note the appearance of moiré in all four bottom squares, in addition to the
third square of the first photo (subtle). Both maze-like and color artifacts
can be seen in the third square of the downsized version. These artifacts
depend on both the type of texture and software used to develop the
digital camera's RAW
file.

MICROLENS ARRAYS

You might wonder why the first diagram in this tutorial did not place each
cavity directly next to each other. Real-world camera sensors do not actually
have photosites which cover the entire surface of the sensor. In fact,
they often cover just half the total area in order to accommodate other electronics.
Each cavity is shown with little peaks between them to direct the photons to
one cavity or the other. Digital cameras contain "microlenses" above each
photosite to enhance their light-gathering ability. These lenses are analogous
to funnels which direct photons into the photosite where the photons would have
otherwise been unused.

Well-designed microlenses can improve the photon signal at each photosite,
and subsequently create images which have less noise for the same exposure time.
Camera manufacturers have been able to use improvements in microlens design
to reduce or maintain noise in the latest high-resolution cameras, despite having
smaller photosites due to squeezing more megapixels into the same sensor area.